In the above-mentioned technical field, patent literature 1 has disclosed a technique of diagnosing an anchor bolt by a natural vibration. Patent literature 2 has disclosed a technique which observes a physical property change surface by using the arrival time of a reflected wave as a clue. Non-patent literature 1 has disclosed a method of observing an amplitude by installing a vibration sensor on a concrete surface instead of an anchor bolt. Non-patent literature 2 has disclosed a technique which observes the amplitude of a hammering sound in order to check peeling of a concrete surface layer.